Wrynecks usually nest in natural crevices, old woodpecker holes, or old swallow holes in banks.
It is a limestone area containing a labyrinth of swallow holes, underground passages and lakes, caverns, and calcareous formations.
Upstream from these "swallow holes" where surface streams are lost to the subsurface, the tributary valleys continue to deepen their channels.
He walks the land, steering clear of the "swallow holes" in the bog because he is afraid of "the pull" of water.
What renown it has derives from being the river traversable by stepping stones at the base of Box Hill, and from its habit, often purported as a reason for its name, of periodically ducking underground into swallow holes.
Due to ongoing strong karstification a typical karst landscape has developed within the Gunung Sewu with characteristic surface (sink and swallow holes, sinking streams, dry valleys, springs etc).
This is not only caused by the local habit of dumping waste directly into karstic structures like swallow holes but also because of insufficient waste water collection and treatment (Fach et al. 2012).
In such environment whenever intense rainfall events cover large areas and rainfall intensity exceeds the discharge capacity of sinks and swallow holes, significant volumes of runoff are produced and stored on surface causing floods and risks for people and goods.
A large karst area of South-Eastern Italy (Puglia) is characterized by endorheic basins, whose runoff does not discharge into the sea but converges toward internal lowlands and infiltrates or flows into underground cave systems through swallow holes.
Recharge in karst either takes place via slow diffuse infiltration which is mainly controlled by matrix dominated features like fractures, epikarst etc. or fast point infiltration via swallow holes or sinking streams which are often directly connected to a conduit system (White 2002; Ford and Williams 2007).
